1. Field of the Invention
The present invention relates to a method for the absolute determination of the position of two objects, which are movable in relation to each other over a defined measuring distance. The present invention further relates to a device for executing the method.
2. Discussion of Related Art
A device in accordance with the species, or a method in accordance with the species, for absolute position determination is known, for example, from FIG. 1 of EP 0 369 031 B1, which corresponds to U.S. Pat. No. 5,121,116, the entire contents of which are hereby incorporated herein by reference. As disclosed in EP 0 369 031 B1, scanning signals with different signal periods are obtained from scanning several measuring graduations with different graduation periods. Among the scanning signals there is also a signal, which only delivers one signal over the measurement distance and is therefore suitable for determining rough absolute position information. The different scanning signals are interpolated and subsequently combined into an absolute position information. The absolute position information has a resolution corresponding to the scanning signals with the highest resolution. A definite relative phase position of the different scanning signals must be assured for the correct determination of an absolute position by the combination of the different scanning signals of differing signal periods. If there is an undefined phase offset between the periodic scanning signals of different signal periods, for example as illustrated in FIG. 5 of EP 0 369 031 B1, an error results when determining the absolute position information. For example, such an undefined phase offset can be the result of tolerances inherent in the production of the scanned measuring graduations, or on the part of the respective scanning units.
For solving these problems it is therefore proposed by EP 0 369 031 B1 to correct the scanning signals of adjacent tracks with regard to an ideal phase position with each other. To this end, phase correction values are applied to the interpolated scanning signals of adjacent measuring graduation tracks. The determination of a suitable correction value for the phase position correction takes place by an iterative method at several sampling times in that overlapping areas of digital position data words in adjacent tracks with different resolution are compared to each other. However, the proposed method for determining correction values requires a not inconsiderable outlay. Moreover, in the presence of a large phase position error at one of the sampling times it is not assured that an accurate correction value can be determined.
It is therefore an object of the present invention to further develop the method in accordance with the species, or the device in accordance with the species, in such a way that a dependable phase position correction between the scanning signals of different measuring graduations is assured with the least possible outlay in circuitry.
This object is attained by a method for the absolute determination of the position of two objects, which are movable in relation to each other over a defined measuring distance. The method includes scanning a first measuring graduation with a first graduation period so as to generate a first periodic scanning signal and scanning a second measuring graduation with a second graduation period so as to generate a second periodic scanning signal, wherein the second graduation period is finer than the first graduation period. Correcting the first scanning signal with respect to its ideal phase position, in that a phase correction value is applied to the first periodic scanning signal and determining the phase correction value for at least a partial section of the measuring distance, wherein the phase correction value is a mean value from a maximum and a minimum phase position deviation of an actual phase position from a setpoint phase position in the at least one partial section.
The above mentioned object is furthermore attained by a device for the absolute position determination of a first object and a second object, which can be moved with respect to each other in a measuring direction. The device includes a first graduation extending in a measuring direction over a defined measuring distance having a first graduation period and a second graduation extending in the measuring direction over the defined measuring distance, having a second graduation period, wherein the second graduation period is finer than the first graduation period, and the first and second graduations are each connected with a first object. A scanning unit, which is connected with a second object for scanning the first and second graduations and generating first and second periodic scanning signals and at least one correcting unit that applies a phase correction value to at least one of the first and second periodic scanning signals, wherein the phase correction value is selected for at least one partial section of the measuring distance as a mean value from a maximum and a minimum phase position deviation of an actual phase position from a setpoint phase position in the at least one partial section.
The steps in accordance with the present invention now assure that a phase position of the scanning signals of adjacent measuring graduations results over the entire measuring distance, which permits a correct determination of the absolute position from these signals. To this end a suitable phase correction value for at least a partial section is determined prior to the measuring operation by a calibration measurement along the entire measuring distance. A phase correction value determined in this way assures that during the actual measuring operation a maximally tolerable phase offset between the different first and second scanning signals is not exceeded in this partial section. If the partial section is selected to equal the entire measuring distance, only a single phase correction value is required. Alternatively to this, it is also possible to divide the measuring distance into several partial sections, for each of which separate phase correction values are determined in accordance with the invention and stored as a function of the position. In this case, the measuring distance results from the sum of the different partial sections. During the measuring operation, the readout in accordance with the position of the phase correction values required for the respective partial section is performed from a data storage unit, which is organized in the form of a table.
The device required for this is constructed of simple circuitry and can also be expanded, depending on the demands made on the measuring system.
The process in accordance with the present invention can be employed in connection with the most diverse measuring systems and in particular is not limited to a particular scanning principle. Furthermore, the present invention can be employed in connection with linear measuring systems, as well as rotational measuring systems.
Further advantages, as well as details, of the present invention ensue from the following description of an exemplary embodiment by the attached drawings.